JPH04330702A - Rare-earth permanent magnet of excellent corrosion resistance - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of the title magnet by a method wherein a texture in a sintered body is constituted of the following: a ferromagnetic main phase; a nonmagnetic B-rich phase; and a nonmagnetic R-rich phase in which the intermetallic compound by rare-earth elements and by carbon is smaller than a specific amount. CONSTITUTION:A permanent magnet alloy composed of the following is formed as a sintered body: rare-earth elements R; boron B; and iron as their residual part. Out of them, the magnetic alloy component R contains 10 to 25 atomic % of one or more kinds out of individual rare-earth element. B is at 1 to 20 atomic %; Fe is used as the residual part excluding the R and the B. A ferromagnetic main phase is composed of R2Fe14B; a nonmagnetic B-rich phase is composed of R1+nFe4B4 [where (n)=0 to 0.5]; and a nonmagnetic R-rich phase is composed of Fe whose content is within a range of 0 to 10wt.%. The intermetallic compound by R and by C in the nonmagnetic R-rich phase is suppressed to 1.0wt.% or lower. Thereby, the corrosion resistance of the title magnet is improved, and it is possible to prevent the density of the sintered body from being lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a rare earth permanent magnet useful in the electrical and electronic fields.

(Prior art) R-Fe-B rare earth permanent magnets have higher magnetic properties than R-Co rare earth permanent magnets, and have a maximum energy product (
(hereinafter referred to as (BH)max), the basic composition of Nd15Fe77B8 reaches up to 35 MGOe, and although the composition has been improved, magnets of 37 MGOe are provided at the mass production level. Currently, high-performance magnets with (BH) max of 40 MGOe or more are being developed, which greatly exceeds the 33 MGOe obtained with R-Co magnets. In addition, the Curie temperature is improved by replacing a part of Fe with Co, Al, Bi, Zr, Hf, V, W, Mo, Cr,
It is known that addition of Ta, Sb, Ge, Nb, Ni, Ti, Sn, etc. improves iHc. Even with R-Fe-B rare earth permanent magnets that have such high properties, the magnet raw material powder in powder form is very easily oxidized. The disadvantage was that the process had to be carried out in a non-oxidizing gas such as nitrogen, or in an inert gas such as argon, or in an organic solvent such as hexane. Furthermore, these R-Fe-B permanent magnet alloy sintered bodies are known to be chemically unstable, and R
-Co-based rare earth permanent magnets have the disadvantage of being more prone to rust.

(Problems to be Solved by the Invention) In order to eliminate the above-mentioned drawbacks, methods such as coating the magnet surface with synthetic resin and plating with metal elements such as Ni and Cu have been adopted. However, since the corrosion resistance of the magnet base itself is not improved, surface treatment alone has its limits, and is not always a satisfactory method.

An object of the present invention is to provide permanent magnets with excellent corrosion resistance by improving the structure inside the magnets in order to improve the corrosion resistance of these magnets themselves.

(Means for Solving the Problem) As a result of repeated research on magnet composition in order to solve such problems, the present inventor found that impurity carbon forms a rare earth element-carbon intermetallic compound, which causes oxidation. promote,
They discovered that this was the cause of magnet deterioration, studied measures to prevent carbon from being mixed in, and completed the present invention. The gist of the present invention is that: 10 to 25 atomic % of a rare earth element R (here, R is at least one or two or more rare earth elements including Nd), 1 to 20 atomic % of boron B, and the balance is iron. The structure in the sintered body of the permanent magnet alloy is a ferromagnetic main phase R2
It has a three-phase structure of Fe14B, a non-magnetic B-rich phase and a non-magnetic R-rich phase, and the intermetallic compound R-C of rare earth elements and carbon in the structure of the non-magnetic R-rich phase is 1.0% by weight.
A rare earth permanent magnet with excellent corrosion resistance is characterized by:

The present invention will be explained in detail below.

The composition of the rare earth iron-based permanent magnet of the present invention is the rare earth element R (
However, R is at least one rare earth element (including Nd), boron (B), and the balance is iron (F
e) is a permanent magnet metal-containing sintered body consisting of Y, La, Ce, and P containing Nd as the magnet alloy component R.
r, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er
, Tm, Yb, and Lu, it is necessary to contain 10 to 25 at% of at least one rare earth element, and if it is less than 10 at%, the α- Precipitation of Fe occurs, and if it exceeds 25 atomic %, a practical magnetic flux density cannot be obtained. B is 1 to 20 atomic%
is an essential requirement, and if it is less than 1 atomic %, the soft magnetic phase of R2Fe
17 compounds are precipitated and the magnetic properties are deteriorated, and if it exceeds 20 atomic %, a sufficient magnetic flux density cannot be obtained due to the precipitation of non-magnetic R1Fe4B4. The remainder excluding R and B may be Fe, and Fe can be replaced by Co, Al, Nb, Zr, Mo, Ga.
, Ti, V, Ni, Si, Bi, Hf, W, Cr, Ta
, Sb, Ge, Sn, etc. by 0 to 20 atomic % substitution, and O, N
, H, Cl, F, S, etc., are optional as long as they do not impair the magnetic properties. In addition, the ferromagnetic main phase is R2Fe14B, and the non-magnetic B-rich phase is R1+n
Fe4B4 (where n=0 to 0.5), the nonmagnetic R-rich phase has an Fe content in the range of 0 to 10% by weight.

It has been known that the magnet sintered body of R-Fe-B permanent magnet alloys is affected in various ways by impurities contained in rare earth elements and other raw material elements. In particular, if a large amount of carbon, an impurity contained in rare earth metals used as raw material elements, or carbon contained in iron or boron elements, which are raw material elements, is mixed in a large amount, the coercive force may be significantly reduced, or the sintered body may deteriorate during sintering. It was found that it was difficult to increase the density.

As a result of detailed research into the influence of impurity carbon on rare earth permanent magnets, the inventor found that when a large amount of impurity carbon is mixed, iron is removed from the non-magnetic R-rich phase in the magnet sintered body. It has been found that an R-rich phase containing carbon and an R-rich phase containing carbon exist separately. R containing this carbon
As a result of analyzing the rich phase using an electron beam microprobe analyzer, it was found that an intermetallic compound R-C was formed between rare earth elements and carbon, and its quantity was proportional to the total carbon content in the sintered body. There is a tendency to increase. It was also found that this intermetallic compound was hardly observed in the raw material ingot and was generated during the heat treatment after sintering. Furthermore, as a result of conducting environmental tests on magnets to investigate the corrosion resistance of this intermetallic compound, it was found that this phase is preferentially corroded and that the corrosion occurs rapidly.

Therefore, it has been found that the precipitation of this phase in the magnet sintered body greatly contributes to the corrosion resistance of the magnet base itself, and is the biggest factor in significantly reducing the corrosion resistance.

The present invention solves these problems by selecting impurities in the elements used as raw materials for rare earth magnets, particularly rare earth elements, transition metal elements, boron, etc. whose carbon concentration is suppressed to below a predetermined limit.

The allowable amount of impurity carbon is calculated based on the fact that the intermetallic compound of rare earth elements and carbon in the structure of the nonmagnetic R-rich phase must be suppressed to 1.0% by weight or less as a result of the magnet's environmental test. By suppressing the carbon content in the permanent magnet alloy composition to 0.04% by weight or less, corrosion resistance was significantly improved and a decrease in sintered body density was also prevented.

Therefore, the amount of carbon contained in the starting element is 0.02% by weight for low carbon rare earth metals and 0.0% by weight for high carbon ones.
It is preferably about 8% by weight or less, and if ferroboron is used, it is necessary to select a high carbon material of about 2% by weight and a low carbon material of about 0.2% by weight. Also, electrolytic iron is highly preferred since it has a content of about 0.01% by weight or less. Furthermore, during press forming in a magnetic field after pulverization, a lubricant containing carbon is sometimes mixed into the magnet fine powder to improve the magnetic field orientation of the compact. It is necessary to reduce the amount of carbon.

These permanent magnet materials are manufactured by known powder metallurgy methods. That is, the raw material element ingot is melted, pulverized, formed in a magnetic field, heat treated, and processed. The raw material element ingot is melted by high frequency in argon or vacuum, and the rare earth element is added last. The obtained magnetic alloy ingot is pulverized into coarse pulverization and fine pulverization. Coarse pulverization is performed using a stamp mill, jaw crusher, brown mill, etc., and fine pulverization is performed using a jet mill, ball mill, etc. In order to prevent oxidation, both are carried out in a non-oxidizing atmosphere, and organic solvents and inert gases are also used. Molding is carried out in a magnetic field by press molding, and the molded body is heated at a predetermined temperature within the range of 1,000 to 1,200°C in a vacuum, an inert gas such as argon or nitrogen, or a non-oxidizing atmosphere. A rare earth permanent magnet with excellent corrosion resistance can be obtained by holding the magnet at a temperature of 30 to 120 minutes for sintering, and then heat-treating the magnet at a temperature of 350° C. to sintering temperature for 30 minutes to 4 hours.

Hereinafter, specific embodiments of the present invention will be described with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

(Example 1, Comparative Example 1) As a starting material, two types of Nd with a purity of 99.7% by weight or more, with a carbon content of 0.082% by weight and 0.412% by weight, were used. .9% by weight of electrolytic iron and 99.5% by weight of boron were used. These raw materials are high-frequency melted, then cast into a mold, and Nd15Fe7
An ingot having a composition of 7B8 was obtained. The carbon content of the obtained ingot was determined from the analysis results of the non-dispersive infrared method.
0.030% by weight (as Example 1) and 0.1%, respectively
It was 41% by weight (referred to as Comparative Example 1). This ingot was coarsely crushed to 32 mesh or less using a jaw crusher and a brown mill, and then finely crushed using a jet mill in a nitrogen stream to obtain raw material magnet powder with an average particle size of about 3 μm.

Next, a magnet sintered body was obtained by a powder metallurgy method. The molding was carried out in a magnetic field by mold press molding, and the size was 20 mmW x 3.
A molded body having a size of 0 mml×10 mm and an apparent density of 4.0 gr/cc was obtained. Next, this molded body was heated to 1,100°C in a vacuum.
The magnet was held at 600° C. for sintering, and then heat treated at 600° C. for 120 minutes to obtain a rare earth permanent magnet. When the R-rich phase of the sintered bodies with these two types of carbon contents was observed using an electron beam microprobe analyzer, it was found that Comparative Example 1 contained a large amount of R-C intermetallic compounds and a small amount of R-C intermetallic compounds in the R-rich phase. An R-rich phase containing Fe was observed, and in Example 1, only an R-rich phase containing Fe was observed. The photographs are shown in FIG. 1 for Example 1 and in FIG. 2 for Comparative Example 1. here,(
a) is a composition image, and (b) is an X-ray image of carbon. To explain this photograph, (a) the contrast of the composition image appears white when the average atomic number of the phase is large, and appears black when it is small.

The white part in the photograph is the R-rich phase, and the other parts are the main phase. (b) The X-ray image is obtained by spectroscopy of the X-rays emitted when the sample is irradiated with an electron beam. Therefore, in this case, the Kα rays of carbon are spectrally analyzed and only the X-rays are obtained and imaged, and the white glowing points are the parts where carbon is present. Table 1 shows the magnetic properties and environmental test results of these sintered bodies. Environmental tests were carried out on these sintered bodies at 120°C for 2
The test was carried out under conditions of atmospheric pressure and 100% relative humidity. The sample of Comparative Example 1, which had a high carbon content, crumbled into pieces when subjected to an environmental test. Further, in Example 1, oxides were only visible on the surface and no collapse was observed.

(Example 2, Comparative Example 2) As starting materials, Nd and Dy with a purity of 99.7% by weight or more, where the carbon content of all rare earth elements is 0.042
Electrolytic iron and cobalt with a purity of 99.9% by weight and boron with a purity of 99.5% by weight were used. , Al, and Nb, and cast them into a mold by high-frequency melting to produce Nd14.05Dy0.74Fe72.95C.
An ingot having a composition of o3.84B7Al1Nb0.4 was obtained. The carbon content of the obtained ingots was 0.021% by weight (Example 2) and 0.122% by weight (Comparative Example 2), respectively, based on the results of non-dispersive infrared analysis. This ingot was processed in the same process as in Example 1 to obtain a magnet sintered body, and the structure was observed using an electron beam microprobe analyzer. As a result, the phase state of the R-rich phase in the sintered body was determined to be due to the additive element. However, the only difference between Example 2 and Comparative Example 2 is the presence or absence of the R-C intermetallic compound. Table 2 shows the phase states of each. Further, the magnetic properties and the results of the environmental test are shown in Table 3.

(Effect of the invention) By reducing impurity carbon in the magnet alloy ingot according to the present invention, the structure of the magnet sintered body made by crushing, molding, sintering, and heat treatment contains R, which has extremely poor corrosion resistance.
-C There is no intermetallic compound, no deterioration in density or orientation is observed, the corrosion resistance of the magnet base itself is improved, and a rare earth permanent magnet with excellent magnetic properties and constant quality can be obtained, and its use in industry. The value is extremely high.

[Brief explanation of the drawing]

Figures 1 (a) and (b) show (a) a composition image and (b) an X-ray image of carbon of the Fe-containing R-rich phase of the permanent magnet alloy of Example 1 of the present invention, taken with an electron beam microprobe analyzer. The photographs, FIGS. 2(a) and 2(b) are photographs corresponding to Example 1 of Comparative Example 1.

Claims (2)

[Claims] Claim 1: Rare earth element R 10 to 25 atomic % (herein R
is at least one or two rare earth elements including Nd
The structure in the sintered body of a permanent magnet alloy consisting of 1 to 20 at. phase structure, and the intermetallic compound R-C of the rare earth element and carbon in the nonmagnetic R-rich phase structure is 1
．． A rare earth permanent magnet with excellent corrosion resistance characterized by a content of 0% by weight or less. [Claim 2] Permanent magnet alloy composition is R10 to 25 atomic %.
, comprising 1 to 20 atomic % of boron and the balance iron, and having an excellent corrosion resistance according to claim 1 or 2, characterized in that the carbon content in the alloy is 0.04% by weight or less. Rare earth permanent magnet.